1. Field of the Invention
This invention relates to a dichromatic coloring matter for a colored liquid crystal display element, particularly for a guest-host type colored liquid crystal display element.
2. Description of the Prior Art
Various dichromatic coloring matters are known and principally of the azo, azomethine or anthraquinone type. Dichromatic coloring matters are required to have, among others, a high dichromatic ratio (CR), a high molecular extinction coefficient, a long life and a high degree of solubility in a liquid crystal. The azo and azomethine dyes generally have a high CR value, but a short life. The anthraquinone dyes have a long life, but a relatively low CR value and a lower molecular extinction coefficient than the azo and azomethine dyes. Therefore, none of these three types of dyes are satisfactory for guest-host type colored liquid crystal display elements. The CR value is particularly important, as it determines the contrast of a display obtained by a guest-host type colored liquid crystal display element. Although the known azo and azomethine dyes are said to have a high CR value, they have a maximum CR value of only 12. A still higher CR value is essential, and it has been desirable to develop a dichromatic coloring matter having a CR value of at least 13.